A period is the part of the menstrual cycle when a woman bleeds from her vagina for a few days. For most women this happens around every 28 days, but it's common for periods to be more or less frequent than this, ranging from every 21 days to every 35 days. period, epoch, era, age mean a division of time. period may designate an extent of time of any length. epoch applies to a period begun or set off by some significant or striking quality, change, or series of events. era suggests a period of history marked by a new or distinct order of things. Triggered by falling progesterone levels, menstruation (commonly referred to as a period ) is the cyclical shedding of the lining, and is a sign that pregnancy has not occurred. Each cycle occurs in phases based on events either in the ovary (ovarian cycle) or in the uterus (uterine cycle). Menstruation — aka having your period — is when blood and tissue from your uterus comes out of your vagina. It usually happens once every month. Understand how to track your menstrual cycle and what to do about irregularities. Do you know when your last menstrual period began or how long it lasted? If not, it might be time to start paying attention. When you track your menstrual cycles, you learn what's typical for you. What is a normal menstrual cycle? Your period — and the PMS symptoms that can accompany it — is just one part of your monthly menstrual cycle. Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). Menstruation (or period) happens monthly. As the uterus sheds its lining, vaginal bleeding follows. Menstrual blood is made of blood and tissue. Lots of girls have cramps with their period, mostly in the first few days. This happens when the uterus (the place inside a woman’s body where a baby grows) squeezes. PERIOD definition: 1. a length of time: 2. a fixed time during the life of a person or in history: 3. in school, a…. Learn more.